Laboratory
1
Ethernet
A Direct Link Network with Media Access Control
Objective
This lab is designed to demonstrate the operation of the Ethernet network. The simulation
in this lab will help you examine the performance of the Ethernet network under different
scenarios.
Overview
The Ethernet is a working example of the more general Carrier Sense, Multiple Access
with Collision Detect (CSMA/CD) local area network technology. The Ethernet is a
multiple-access network, meaning that a set of nodes sends and receives frames over a
shared link. The  carrier sense in CSMA/CD means that all the nodes can distinguish
between an idle and a busy link. The  collision detect means that a node listens as it
transmits and can therefore detect when a frame it is transmitting has interfered (collided)
with a frame transmitted by another node. The Ethernet is said to be a 1-persistent
protocol because an adaptor with a frame to send transmits with probability 1 whenever a
busy line goes idle.
In this lab you will set up an Ethernet with 14 nodes connected via a coaxial link in a bus
topology. The coaxial link is operating at a data rate of 10 Mbps. You will study how the
throughput of the network is affected by the network load as well as the size of the
packets.
Procedure
Create a New Project
To create a new project for the Ethernet network:
1. Start OPNET IT Guru Academic Edition Ò! Choose New from the File menu.
2. Select Project Ò! Click OK Ò! Name the project _Ethernet, and
the scenario Coax Ò! Click OK.
Local area networks
3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty
(LANs) are designed to
Scenario is selected Ò! Click Next Ò! Choose Office from the Network Scale list
span distances of up to a
few thousand meters. Ò! Click Next Ò! Assign 200 to X Span and keep Y Span as 100 Ò! Click Next
twice Ò! Click OK.
4. Close the Object Palette dialog box.
Create the Network
To create our coaxial Ethernet network:
1. To create the network configuration, select Topology Ò! Rapid Configuration.
From the drop-down menu choose Bus and click OK.
2. Click the Select Models button in the Rapid Configuration dialog box. From the
Model List drop-down menu choose ethcoax and click OK.
3. In the Rapid Configuration dialog box, set the following eight values and click OK.
The eth_tap is an
Ethernet bus tap that
connects a node with the
bus.
The eth_coax is an
Ethernet bus that can
connect nodes with bus
receivers and
transmitters via taps.
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4. To configure the coaxial bus, right-click on the horizontal link Ò! Select Advanced
Edit Attributes from the menu:
a. Click on the value of the model attribute Ò! Select Edit from the drop-
down menu Ò! Choose the eth_coax_adv model.
b. Assign the value 0.05 to the delay attribute (propagation delay in sec/m).
c. Assign 5 to the thickness attribute.
d. Click OK.
A higher delay is used
here as an alternative to
generating higher traffic
which would require
much longer simulation
time.
Thickness specifies the
thickness of the line used
to  draw the bus link.
5. Now you have created the network. It should look like the illustration below.
6. Make sure to save your project.
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Configure the Network Nodes
To configure the traffic generated by the nodes:
1. Right-click on any of the 30 nodes Ò! Select Similar Nodes. Now all nodes in the
network are selected.
2. Right-click on any of the 30 nodes Ò! Edit Attributes.
3. Check the Apply Changes to Selected Objects check box. This is important to
avoid reconfiguring each node individually.
4. Expand the Traffic Generation Parameters hierarchy:
a. Change the value of the ON State Time to exponential(100) Ò! Change
The argument of the
the value of the OFF State Time to exponential(0). (Note: Packets are
exponential distribution
generated only in the "ON" state.)
is the mean of the
interval between
successive events. In the
5. Expand the Packet Generation Arguments hierarchy:
exponential distribution
the probability of
a. Change the value of the Packet Size attribute to constant(1024).
occurrence of the next
event by a given time is
b. Right-click on the Interarrival Time attribute and choose Promote
not at all dependent
Attribute to Higher Level. This allows us to assign multiple values to
upon the time of
occurrence of the last
the Interarrival Time attribute and hence to test the network
event or the elapsed time
performance under different loads.
since that event.
The interarrival time is
the time between
successive packet
generations in the "ON"
state.
6. Click OK to return back to the Project Editor.
7. Make sure to save your project.
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Configure the Simulation
To examine the network performance under different loads, you need to run the simulation
several times by changing the load into the network. There is an easy way to do that.
Recall that we promoted the Interarrival Time attribute for package generation. Here we
will assign different values to that attribute:
1. Click on the Configure/Run Simulation button:
2. Make sure that the Common tab is chosen Ò! Assign 15 seconds to the
Duration.
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3. Click on the Object Attributes tab.
4. Click on the Add button. The Add Attribute dialog box should appear filled with
the promoted attributes of all nodes in the network (if you do not see the attributes
in the list, close the whole project and reopen it). You need to add the Interarrival
Time attribute for all nodes. To do that:
a. Click on the first attribute in the list (Office Network.node_0.Traffic
Generation & .) Ò! Click the Wildcard button Ò! Click on node_0 and
choose the asterisk (*) from the drop-down menu Ò! Click OK.
b. A new attribute is now generated containing the asterisk (the second one in
the list), and you need to add it by clicking on the corresponding cell under
the Add? column.
c. The Add Attribute dialog box should look like the following. Click OK.
5. Now you should see the Office Network.*.Traffic Generation Parameter & in
the list of simulation object attributes. Click on that attribute to select it Ò! Click the
Values button of the dialog box.
6. Add the following nine values. (Note: To add the first value, double-click on the
first cell in the Value column Ò! Type  exponential (2) into the textbox and hit
enter. Repeat this for all nine values.)
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7. Click OK. Now look at the upper-right corner of the Simulation Configuration
dialog box and make sure that the Number of runs in set is 9.
8. For each simulation of the nine runs, we need the simulator to save a  scalar
value that represents the  average load in the network and to save another
scalar value that represents the average throughput of the network. To save
these scalars we need to configure the simulator to save them in a file. Click on
the Advanced tab in the Configure Simulation dialog box.
9. Assign _Ethernet_Coax to the Scalar file text field.
10. Click OK and then save your project.
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Choose the Statistics
To choose the statistics to be collected during the simulation:
1. Right-click anywhere in the project workspace (but not on one of the nodes or
links) and select Choose Individual Statistics from the pop-up menu Ò! Expand
the Global Statistics hierarchy.
a. Expand the Traffic Sink hierarchy Ò! Click the check box next to Traffic
Received (packets/sec) (make sure you select the statistic with units of
packets/sec),
b. Expand the Traffic Source hierarchy Ò! Click the check box next to
Traffic Sent (packets/sec).
c. Click OK.
2. Now to collect the average of the above statistics as a scalar value by the end of
each simulation run:
a. Select Choose Statistics (Advanced) from the Simulation menu.
A probe represents a
b. The Traffic Sent and Traffic Received probes should appear under the
request by the user to
Global Statistic Probes.
collect a particular piece
of data about a
simulation.
c. Right-click on Traffic Received probe Ò! Edit Attributes. Set the scalar
data attribute to enabled Ò! Set the scalar type attribute to time
average Ò! Compare to the following figure and click OK.
d. Repeat the previous step with the Traffic Sent probe.
e. Select save from the File menu in the Probe Model window and then
close that window.
f. Now you are back to the Project Editor. Make sure to save your project.
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Run the Simulation
To run the simulation:
1. Click on the Configure/Run Simulation button: Ò! Make sure that 15
second(s) (not hours) is assigned to the Duration Ò! Click Run. Depending on
the speed of your processor, this may take several minutes to complete.
2. Now the simulator is completing nine runs, one for each traffic generation
interarrival time (representing the load into the network). Notice that each
successive run takes longer to complete because the traffic intensity is increasing.
3. After the nine simulation runs complete, click Close.
4. Save your project.
When you rerun the simulation, OPNET IT Guru will  append the new results to the
results already in the scalar file. To avoid that, delete the scalar file before you start a new
run. (Note: Deleting the scalar file after a run will result in losing the collected results from
that run.)
" Go to the File menu Ò! Select Model Files Ò! Delete Model Files Ò!
Select ( .os): Output Scalars Ò! Select the scalar file to be deleted; in this lab it
is _Ethernet_Coax_Scalar Ò! Confirm the deletion by clicking
OK Ò! Click Close.
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View the Results
To view and analyze the results:
1. Select View Results (Advanced) from the Results menu. Now the Analysis
Configuration tool is open.
2. Recall that we saved the average results in a scalar file. To load this file, select
Load Output Scalar File from the File menu Ò! Select initials>_Ethernet-Coax from the pop-up menu.
3. Select Create Scalar Panel from the Panels menu Ò! Assign Traffic
Source.Traffic Sent (packets/sec).average to Horizontal Ò! Assign Traffic
Sink.Traffic Received (packets/sec).average to Vertical Ò! Click OK.
4. The resulting graph should resemble the one below:
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Further Readings
- OPNET Ethernet Model Description: From the Protocols menu, select Ethernet
Ò! Model Usage Guide.
Questions
1) Explain the graph we received in the simulation that shows the relationship
between the received (throughput) and sent (load) packets. Why does the
throughput drop when the load is either very low or very high?
2) Create three duplicates of the simulation scenario implemented in this lab. Name
these scenarios Coax_Q2a, Coax_Q2b, and Coax_Q2c. Set the Interarrival
Time attribute of the Packet Generation Arguments for all nodes (make sure to
check Apply Changes to Selected Objects while editing the attribute) in the
new scenarios as follows:
- Coax_Q2a scenario: exponential(0.1)
- Coax_Q2b scenario: exponential(0.05)
- Coax_Q2c scenario: exponential(0.025)
In all the above new scenarios, open the Configure Simulation dialog box and
from the Object Attributes delete the multiple-value attribute (the only attribute
shown in the list).
Choose the following statistic for node 0: Ethcoax Collision Count. Make sure
that the following global statistic is chosen: Global StatisticsTraffic
SinkTraffic Received (packet/sec). (Refer to the Choose the Statistics section
in the lab.)
Run the simulation for all three new scenarios. Get two graphs: one to compare
node 0 s collision counts in these three scenarios and the other graph to compare
the received traffic from the three scenarios. Explain the graphs and comment on
the results. (Note: To compare results you need to select Compare Results from
the Results menu after the simulation run is done.)
3) To study the effect of the number of stations on Ethernet segment performance,
create a duplicate of the Coax_Q2c scenario, which you created in Question 2.
Name the new scenario Coax_Q3. In the new scenario, remove the odd-
numbered nodes, a total of 15 nodes (node 1, node 3, & , and node 29). Run the
simulation for the new scenario. Create a graph that compares node 0 s collision
counts in scenarios Coax_Q2c and Coax_Q3. Explain the graph and comment
on the results.
4) In the simulation a packet size of 1024 bytes is used (Note: Each Ethernet packet
can contain up to 1500 bytes of data). To study the effect of the packet size on
the throughput of the created Ethernet network, create a duplicate of the
Coax_Q2c scenario, which you created in Question 2. Name the new scenario
Coax_Q4. In the new scenario use a packet size of 512 bytes (for all nodes). For
both Coax_Q2c and Coax_Q4 scenarios, choose the following global statistic:
Global StatisticsTraffic SinkTraffic Received (bits/sec). Rerun the
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simulation of Coax_Q2c and Coax_Q4 scenarios. Create a graph that compares
the throughput as packets/sec and another graph that compares the throughput
as bits/sec in Coax_Q2c and Coax_Q4 scenarios. Explain the graphs and
comment on the results.
Lab Report
Prepare a report that follows the guidelines explained in Lab 0. The report should include
the answers to the above questions as well as the graphs you generated from the
simulation scenarios. Discuss the results you obtained and compare these results with
your expectations. Mention any anomalies or unexplained behaviors.
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